In the method of forming electrophotographic images employing an electrostatic latent image developing toner (hereinafter, simply referred to as a toner), an electrostatic latent image corresponding to a pattern of the original document (that is, image information of the original document) is formed by irradiating image-wise light on a photoreceptor, after which the resulting latent image is developed with a toner to form an image.
In particular, in recent years, color images have been widely used. In case of the color images, a method has been practically used such that a toner image composed of individual elementary colors is formed by developing the electrostatic latent images corresponding the respective elementary colors employing corresponding color toners, followed by superposing the above elementary color images. Color toners for forming above-described color image include a yellow toner, a magenta toner, a cyan toner and a black toner, each of which contains a binding resin (a binder resin) which is a thermoplastic resin and a colorant of each color.
Namely, the method of forming electrophotographic images is described as follows. In general, an electrostatic latent image is formed by irradiating a photoreceptor with light having image information of the original document via various methods on a photoreceptor having a light conductive material, and the resulting electrostatic latent image formed on the photoreceptor is developed by a charged toner to form a toner image, which is then transferred to an image transper material (for example, a plain paper, or intermediate transfer body), to obtain a final visual image through a fixing step employing a heat fixing device.
In order to form a full color image, the image information of the original image is decomposed into image patterns of yellow, magenta, cyan and black. A latent image of each of the yellow, magenta, cyan and black image patterns is formed on a photoreceptor, which is then developed with a toner of corresponding color.
As a colorant constituting a toner, commonly known organic pigments and oil soluble dyes have been employed, and toners in which either an organic pigment or an oil soluble dye is employed, or toners in which mixture of an organic pigment and an oil soluble dye is employed, have been proposed. However, each of the organic pigment and the oil soluble dye, both of which are employed as the colorants, has various problems.
Specifically, the organic pigment generally exhibits excellent heat resistance and light stability compared to the oil soluble dye, but exhibit a problem such that, since the organic pigment exist in a toner as dispersed particles, the organic pigment exhibits strong hiding power resulting in decrease in transparency of a toner. Further, since the pigment exhibits a low dispersibility, the low dispersibility of the pigment results in further lower transparency, lower color saturation and a poor color reproducibility in the formed toner images.
In particular, in a full-color image which is formed by superposing toner images of each single color, since an image in the lowest layer of the superposed toner images is hidden by images in the upper layers, the aforesaid toner image in the lowest layer tends to be less-visible and unclear. In order to prevent the above problem, the toner of each color is desired exhibit transparency in the fixed state, and therefore, in order to obtain excellent color reproduction, colorants constituting the toner are desired to exhibit high dispersibility.
In the field of printers, along with the recent progress in electrophotographic method, a method of forming electrophotographic images employing a toner have been widely used. In addition to conventional monochrome prints represented by document preparation, full-color prints have, in recent years, become widely. Since such a full-color image forming apparatus can make on demand as many prints as necessary without making proofs as in printing industry, full-color image forming apparatuses have been widely utilized in the light-printing field which has many opportunities to take orders of a small amount of prints (for example, refer to Patent Document 1).
In particular, in order to make full-color prints, such as catalogues and advertisements employing toners, toners used for the prints are desired to exhibit excellent color reproduction so as to provide images faithful to the original images. In order to realize it, various colorants have been studied for the purpose of improving color reproduction of color toners.
For example, as a cyan colorant, colorants having a structure of a primary framework of copper phthalocyanine have been widely employed, but these colorants tend to exhibit inferior in color saturation. As a cyan colorant exhibiting high color saturation, squarelium compounds have been known, but these compounds exhibited poor separation properties at the time of fixing, and image defects due to wax unevenness even when prints are normally discharged. Also squarelium compounds tend to exhibit document offsetting.
Patent Document 1: Japanese Patent Application Publication No. 2008-64940